1. Field of the Invention
This invention relates to a shell-form transformer and, more particularly, to a shell-form transformer for charging an energy storage device in a battery powered impact device in which the primary and secondary windings are wound in a predetermined manner around a center leg having an air gap therein.
2. Description of the Prior Art
Transformers are capable of stepping up or stepping down alternating voltages. As is elementary, two or more coils, each having a plurality of turns, are arranged in the transformer so that mutual inductance exist between the coils. Energy is transferred from a primary winding or coil to a secondary winding or coil by a mutual magnetic field. A core, typically fabricated from an iron alloy, links the windings and provides a high-permeance path for the mutual magnetic flux.
The iron-alloy core is normally made up of a plurality of laminations in which each lamination is insulated from the others by an insulating coating, such as iron oxide. The laminations prevent the formation of large eddy currents by reducing the paths for such currents. Without such laminations, the resultant eddy currents would cause excessively high core heating and a significant reduction in transformer efficiency.
FIG. 1 illustrates a shell-form transformer 1 according to the prior art in which primary and secondary windings 10 are wound on a center leg 15 of a core 5. As shown, no windings are wound around the outer two legs 20 and 25. Typically, the low-voltage winding is wound closer to the center leg so as to minimize the amount of insulating material required for the coils. In core 5, mutual flux flows through center leg 15 and each of the outer two legs 20 and 25 serves as a return path for half of the mutual flux. As such, the cross-sectional area of center leg 15 is approximately twice that of each outer leg 20 and 25.
Battery powered devices, for example, battery powered electric staplers or the like, use transformers to step-up an applied voltage to a predetermined value. For example, in a pending U.S. Patent Application assigned to the present Assignee, entitled "Apparatus for Driving the Armature of an Electric Stapler", S. Goldner, Ser. No. 07/486,247, filed on Feb. 28, 1990, which is incorporated herein by reference, a step-up transformer is utilized to charge an energy storage device or capacitor which, when fully charged and triggered, drives an armature causing a staple to discharge. In this application, it is desirable to transform an applied input voltage of approximately 5-15 volts to a voltage level in the range of approximately 150 to 300 volts. It is to be appreciated, that by maximizing the transformer output voltage as described, the time required to charge the capacitor is minimized which, in turn, enables rapid continuous discharging of the staples.
The output voltage of a transformer is typically increased by increasing the applied current. However, the maximum current which can be applied to a transformer is limited by core saturation, wherein further increases in the applied current fail to produce an increase in the core flux density. As is to be appreciated, core saturation reduces the transformer operating efficiency and limits the output voltage from the transformer secondary. Therefore, to increase the transformer output voltage beyond the limit imposed by core saturation (which would increase the charging rate of the capacitor used in the electric stapler of the referenced application), the size of the core must be increased. As a result, the size and cost of the transformer, and thus the electric stapler, are increased. The prior art has failed to provide a transformer having a satisfactorily small size which is inexpensive and, when used with an electric stapler or the like, enables rapid charging of an energy storage device therein.